Modulation of motor and premotor cortices by actions, action words and action sentences

Recent research has indicated that processing different kinds of action verbs, such as those related to arm or leg movements (e.g. grab, kick), engages regions along the motor strip responsible for the execution of the corresponding actions. It has been proposed that this activation reflects action-related meaning and that these regions are automatically triggered whenever action words are encountered. However, this view is not universally shared by cognitive studies that have shown that the representation of verbs is highly dependent on the interactions with the semantic context. We investigated these views in a set of fMRI studies, in which participants performed a movement localiser task and listened to arm- and leg-related verbs that were presented in isolation (e.g. kick), in literal sentences (as in kick the ball) and idiomatic sentences (as in kick the bucket). We found significant activation in motor regions when action verbs were presented in isolation, and, to a lesser extent, in literal sentential contexts. When the same verbs were presented in idiomatic contexts, activation was found in fronto-temporal regions, associated with language processing, but not in motor and premotor cortices. These results suggest that motor responses were context-dependent, rather than automatic and invariable. These findings lend support to cognitive theories of semantic flexibility, by showing that the nature of the semantic context determines the degree to which alternative senses and particularly relevant features are processed when a word is heard.